Combined toxicity of endocrine-disrupting chemicals: A review

The combined toxicological assessment provides a realistic approach for hazard evaluation of chemical cocktails that co-existed in the environment. This review provides a holistic insight into the studies highlighting the mixture toxicity of the endocrine-disrupting chemicals (EDCs), especially focusing on the screening of biochemical pathways and other toxicogenetic endpoints. Reviewed literature showed that numerous multiplexed toxicogenomic techniques were applied to determine reproductive effects in vertebrates, but limited studies were found in non-mammalian species after mixture chemical exposure. Further, we found that the experimental design and concentration selection are the two important parameters in mixture toxicity studies that should be time- and cost-effective, highly precise, and environmentally relevant. A summary of EDC mixtures affecting the thyroid axis, estrogen axis, androgen axis, growth stress, and immune system via in vivo bioassays was also presented. It is interesting to mention that majority of estrogenic effects of the mixtures were sex-dependent, particularly observed in male fish as compared to female fish. Further, the androgen axis was perturbed with serious malformations in male rat testis (epididymal or gubernacular lesions, and deciduous spermatids). Also, transgenerational epigenetic effects were promoted in the F₃ and F₄ generations in the form of DNA methylation epimutations in sperm, increasing polycystic ovaries and reducing the offspring. Similarly, increased oxidative stress, high antioxidant enzymatic activities, disturbed estrous cycle, and decreased steroidogenesis were the commonly found effects after acute or chronic exposure to EDC mixtures. Importantly, the concentration addition (CA) and independent action (IA) models became more prevalent and suitable predictive models to unveil the prominence of synergistic estrogenic and anti-androgenic effects of chemical mixtures. More importantly, this review encompasses the research challenges and gaps in the existing knowledge and specific future research perspectives on combined toxicity.